108 Trans. Acad. Sci. of St. Louis. 



Another distinguishing characteristic is the manner in 

 which the whorls of the young shells form in contact. In 

 truncatus the spire is perfectly flat for all of the whorls and 

 the sutures form strong, elevated ridges from the elevation 

 of the carina of the whorl above the dorsal plane. The nu- 

 cleus, also, is simply a slightly elevated oval disk, perfectly in 

 accord with the general truncated aspect of the whole spire. 



In trivolvis the whorls impinge in such a manner as to form 

 a depressed suture, and each whorl, beginning with the second, 

 is raised a trifle above the last. The nucleus is much smaller 

 than that of truncatus and is depressed a little below 

 the first whorl. The whorls of truncatus are all rather tightly 

 coiled, while those of trivolvis are tightly coiled in the young 

 shell (2-4 whorls) but become loosely coiled in the adult. 



It is the opinion of the writer that truncatus is the ances- 

 tral form of trivolvis, and not vice versa, as was stated in the 

 " Molluscaof the Chicago Area," the close relationship being 

 shown in the spiral lines and truncated spire of the young of 

 both species. I have not seen the occurrence of fossil speci- 

 mens of truncatus recorded, but it should certainly be found 

 in the Pleistocene deposits of Michigan. 



The geographical distribution of truncatus is somewhat 

 limited judging by the records, which give the following local- 

 ities: Saginaw Bay, Michigan (Miles); Elk Eiver, Antrim 

 Co., Michigan (Walker); Millers, Indiana (Higley) ; North 

 Branch Chicago River and George Lake (Jensen) ; Chicago 

 Drainage Canal, Summit, Illinois (Baker); Port Austin, 

 Mich. (C. A. Davis); Bayport, Mich. (Mrs. A. C. Lane). 



The species seems to be extremely rare and but a half dozen 

 specimens have rewarded the efforts of the Chicago Conchol- 

 ogists during the past twenty years. It is one of the hand- 

 somest of the Planorbes and it is to be hoped that further 

 search will add new localities and bring a number of specimens 

 to light. 



BIBLIOGRAPHY. 



1860. Miles, Manley. Catalogue of the Mollusks of Michi- 

 gan. Winchell's Rep. Geol. Surv. Mich., 1861, 

 p. 238. 



